# R/daylength.R In weather: Weather data manipulation

#### Documented in daylength

```# Author: Robert J. Hijmans

daylength <- function(lat, doy) {
if (class(doy) == 'Date' | class(doy) == 'character') {
doy <- doyFromDate(doy)
}
lat[lat > 90 | lat < -90] <- NA
doy <- doy %% 365

#Ecological Modeling_, volume 80 (1995) pp. 87-95, called "A Model
#Comparison for Daylength as a Function of Latitude and Day of the Year."
P <- asin(0.39795 * cos(0.2163108 + 2 * atan(0.9671396 * tan(0.00860*(doy-186)))))
a <-  (sin(0.8333 * pi/180) + sin(lat * pi/180) * sin(P)) / (cos(lat * pi/180) * cos(P));
a <- pmin(pmax(a, -1), 1)
DL <- 24 - (24/pi) * acos(a)
return(DL)
}

.daylength2 <- function(lat, doy) {
if (class(doy) == 'Date' | class(doy) == 'character') {
doy <- doyFromDate(doy)
}
lat[lat > 90 | lat < -90] <- NA
doy[doy==366] <- 365
doy[doy < 1] <- 365 + doy[doy < 1]
doy[doy > 365] <- doy[doy > 365] - 365
if (isTRUE(any(doy<1))  | isTRUE(any(doy>365))) {
stop('cannot understand value for doy')
}

# after Goudriaan and Van Laar
#  Sine and cosine of latitude (LAT)
# Maximal sine of declination;}
#{Sine and cosine of declination (Eqns 3.4, 3.5);}
SINDEC <- -SINDCM * cos(2*pi*(doy+10)/365)
COSDEC <- sqrt(1-SINDEC*SINDEC);
#The terms A and B according to Eqn 3.3;}
A <- SINLAT*SINDEC;
B <- COSLAT*COSDEC;
C <- A/B;
#Daylength according to Eqn 3.6; arcsin(c) = arctan(c/sqrt(c*c+1))}
DAYL <- 12* (1+(2/pi)* atan(C/sqrt(C*C+1)))
return(DAYL)
}
```

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weather documentation built on May 31, 2017, 2:41 a.m.